The present invention relates to an electrically drivable motor vehicle and a method for operating a circuit arrangement for such a motor vehicle.
Electrically drivable motor vehicles are increasingly becoming the focus of interest. The arrangement and the equipping of such electric vehicles with corresponding vehicle batteries can give rise to problems. Electrically operated motor vehicles with two vehicle batteries cooperating in case of a corresponding power requirement are known. A simple parallel connection of the two vehicle batteries is prohibited, however, as it is known that parallel connected vehicle batteries constitute an unstable system, wherein reloading of energies, according to the manufacturing tolerance of no-load voltages, can arise. Such a situation becomes even more unstable insofar as the no-load voltages move greatly with the ageing of the batteries or in case of damage to one of the vehicle batteries.
To date, this problem has been solved through the use of so-called DC/DC converters disposed between the vehicle batteries and providing a no-load voltage that can precisely cancel out the no-load difference, whereby the vehicle batteries are equally loaded in a first step even if they have differences in the no-load voltages. Such DC/DC converters can counter control in a compensating way through their counter voltage even when one of the vehicle batteries already has a lower charging state than a second vehicle battery. In this case the counter voltage of the DC/DC converter is controlled by a superordinate operating strategy so that the second vehicle battery is loaded considerably more greatly in partial load operation than the first battery. A stabilization of the whole system can also be achieved and the charge of the two vehicle batteries adapted. Such DC/DC converters are, however, comparatively expensive and require high resources.
The present invention addresses these problems of an electrically drivable motor vehicle with at least two vehicle batteries by use of an improved or at least an alternative embodiment characterized by a different structure and, associated therewith, by clearly lower costs.
This problem is solved according to the invention in that the load interrupter switches respectively comprise two power branches arranged anti-parallel to each other. A reliable charging and discharging operation of each vehicle battery can thereby be guaranteed. This is necessary in an electrically driven vehicle in order to guarantee both a reliable electrical driving operation and also the recovery of kinetic brake energy (recuperation) in a reliable manner.
Exemplary embodiments of the present invention provide an electrically drivable motor vehicle with at least two vehicle batteries that can be connected in parallel and an associated electronic circuit arrangement, whereby this circuit arrangement comprises a number of electronic load interrupter switches corresponding to the number of vehicle batteries, via which the vehicle batteries can be connected through individually or in any combination, in particular together, to an electrical consumer. The electronic circuit arrangement thus replaces the conventional use of a DC/DC converter, whereby not only a considerable cost saving is achieved but also a reduction in the weight and the 3% reduction in electrical power loss that occurs using such a DC/DC converter. In contrast, the inventive electronic circuit arrangement works virtually loss-free, whereby a degree of efficiency achievable overall is considerably higher than with comparable systems known from the prior art. With the inventive circuit arrangement it is thus possible in a simple and at the same time cost effective way to make a plurality of parallel connectable high volt batteries optimally accessible.
In one embodiment at least one load interrupter switch comprises a third power branch which is arranged parallel to the two power branches arranged anti-parallel to each other. Through the third power branch the current carrying capacity of the at least one load interrupter switch is advantageously increased.
In one embodiment of the invention the two power branches arranged anti-parallel to each other respectively comprise an IGBT and a blocking diode. The respective modules can thus be charged and discharged, whereby an undesirable and parasitic current flow between battery modules with different charging states or voltages can be effectively avoided.
In a further embodiment of the invention the third power branch comprises a field effect transistor unit which comprises at least one field effect transistor. The field effect transistor is preferably a power MOSFET or as a MOSFET. The at least one field effect transistor is preferably a low voltage MOSFET.
The use of such a field effect transistor can lead to a clear reduction in the power loss arising on the respective IGBTs. The field effect transistor unit preferably comprises a relay unit comprising at least one relay arranged in series with the at least one field effect transistor.
The field effect transistor unit preferably comprises a plurality of field effect transistors and the relay unit comprises a plurality of relays. Each relay can be electrically conductively connected via a connection to one of the vehicle batteries. Each relay can be electrically conductively connected via a further connection to at least one connection of each field effect transistor. Each field effect transistor can be electrically conductively connected via its further connection to the electrical consumer.
In a further embodiment of the invention the number of relays exceeds the number of field effect transistors. The vehicle batteries are preferably high volt batteries.
According to the invention a method is provided for operating a circuit arrangement with at least two electronic load interrupter switches for an electrically drivable motor vehicle.
In an embodiment of the inventive method at least two electronic load interrupter switches are connected through, whereby the internal resistances of at least two vehicle batteries and the electrical consumer form a star connection. In this embodiment therefore the output power of at least two batteries can be used in order to provide the power requirement necessary for a high load operation. In addition the capacitance of at least two batteries can be exhausted in order to facilitate the high load operation for a sufficient duration.
In a further embodiment the individual vehicle batteries are connected through in partial load operation via corresponding load interrupter switches alternately to the electrical consumer. The vehicle batteries are thus evenly discharged. Voltage differences between the individual vehicle batteries can thus be minimized or compensated advantageously.
The switching between partial load operation and high load operation preferably takes place in dependence upon the difference of the no-load voltages of two vehicle batteries or in dependence upon the differences of the no-load voltages of a plurality of vehicle batteries. Thus, it is possible to prevent, for example, the voltage differences between the batteries from exceeding predetermined threshold values.
In a further embodiment the IGBTs of the load interrupter switches are respectively assigned to a vehicle battery, a main battery and at least one subsidiary battery. For a period of time, the IGBTs of the load interrupter switch of the main battery are permanently connected through. The IGBTs of the load interrupter switch of the respective subsidiary battery are connected through in dependence upon requirements. It is thus provided that the power necessary for the normal driving operation is provided by the main battery. In other operating situations such as for example boost (accelerate) or recuperation operation, the power and the capacitance of at least one subsidiary battery can be additionally used.
A further embodiment provides that, for connection of the field effect transistor unit, first the relay is closed and subsequently the field effect transistor is connected, and then for disconnection of the field effect transistor unit, first the relay is opened and subsequently the field effect transistor is disconnected. An alternative embodiment provides that for the connection of the field effect transistor unit, first the relays are closed and subsequently the field effect transistors are connected, and then for disconnection of the field effect transistor unit, first the relays are opened and then the field effect transistors are disconnected. When disconnecting, therefore, an undesirable voltage drop on the field effect transistors can be avoided. The connection and/or disconnection of the field effect transistor unit is preferably performed during the secure driving operation of the motor vehicle.
In a further embodiment the field effect transistor unit is connected as a reduction means for reducing the temperature of the load interrupter switch.
Further important features and advantages of the invention follow from the drawings and the associated description of the figures by reference to the drawings.
It is understood that the abovementioned features and those to be explained below can be used not only in the indicated combination but also in other combinations or alone without going outside of the scope of the invention.